The present invention relates generally to improvements in portable combustion powered fastener driving tools, and specifically to improvements relating to the retarding of the post-combustion opening of the combustion chamber to allow the piston to properly return to the start position.
Portable combustion powered, or so-called IMPULSE.RTM. brand tools for use in driving fasteners into workpieces are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,552,162; 4,483,473; 4,483,474; 4,403,722 and 5,263,439, all of which are incorporated by reference herein. Similar combustion powered nail and staple driving tools are available commercially from ITW-Paslode of Lincolnshire, Ill. under the IMPULSE.RTM. brand.
Such tools incorporate a generally pistol-shaped tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A powerful, battery-powered electronic power distribution unit produces the spark for ignition, and a fan located in the combustion chamber provides for both an efficient combustion within the chamber, and facilitates scavenging, including the exhaust of combustion by-products. The engine includes a reciprocating piston with an elongate, rigid driver blade disposed within a cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a workpiece contact element at the end of the linkage is pressed against a workpiece. The workpiece contact element is designed to reciprocate relative to a nosepiece, which is fixed to the housing. This pressing action also triggers a fuel metering valve to introduce a specified volume of fuel into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the ignition of a charge of gas in the combustion chamber of the engine, the piston and driver blade are shot downward to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original, or "ready" position through differential gas pressures within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
One of the design criteria for conventional combustion tools is that the trigger cannot be operated until the nosepiece is pressed against the workpiece. This feature delays ignition until the combustion chamber is closed. A suitable trigger lockout mechanism is disclosed in U.S. Pat. No. 4,483,474, which is incorporated by reference. In the '474 patent, a cam and lever mechanism prevent depression of the trigger until the nosepiece is pressed against the workpiece, closing the combustion chamber. Upon firing, the combustion chamber cannot open until the trigger is released.
A recent development in combustion tools is the creation of high energy tools which produce more force for driving the fasteners into the workpiece. In some such tools, the additional force is obtained through the use of an extended cylinder through which the piston travels, thus providing the piston with a longer stroke. In other higher energy designs, the volume of the combustion chamber is increased. In these designs, the increased surface area of the combustion chamber is attempted to be minimized, and the surface area of the cylinder may remain the same. There is more combustion energy, but not equivalently more surface area for cooling and creating the differential pressure to return the piston to the start position. Accordingly, the piston returns more slowly.
In longer length tools, the time required for the return of the piston is increased as the length of the cylinder increases. It has been found that in some relatively recently-developed high energy combustion tools, the piston requires approximately twice as long to return to its start position as in conventional combustion tools having a relatively shorter stroke. Obviously, the tool should not be fired until the piston has been completely returned to the start position.
In combustion tools equipped as described above, in the event that the trigger switch is released and the tool lifted from the workpiece before the piston has returned to its start position, the valve linkage allows the combustion chamber to open, thus destroying the differential gas pressures which assist in the upward return of the piston. In order to have consistent firings, the size of the combustion chamber must always be the same.
Another design criteria for combustion tools of this type is the desire for operators working on construction sites to practice what is commonly referred to as "bump firing". This is a procedure of rapid firing of the tool such that the operator utilizes the recoil of the firing of a first fastener to lift the tool and rapidly place it in position for the next firing. As such, there is a shorter period of time in which the tool is maintained with the nosepiece and the workpiece contact element pressed against the workpiece. To prevent misfires, the tool must be allowed to recover between firings by the piston returning to the start position before a subsequent ignition. For proper piston return, the combustion chamber must remained sealed until the piston reaches the start position.
Accordingly, it is an object of the present invention to provide an improved combustion powered tool which prolongs the scaled condition in the combustion chamber until the piston has returned to its pre-combustion start position.
Another object of the present invention is to provide an improved combustion powered tool which features a mechanism for keeping the combustion chamber closed until the piston returns to its start position.
A further object of the present invention is to provide an improved combustion powered tool wherein the combustion chamber is kept closed until the return of the piston by a mechanism which delays the release of the trigger switch, and, through connection to the lockout mechanism, thus ultimately delays the opening of the combustion chamber.
An additional object of the present invention is to provide an improved combustion powered tool featuring a trigger switch which is relatively easier to depress or activate than it is to return to its initial, non-activated position.
Still another object of the present invention is to provide an improved combustion powered tool featuring a lockout mechanism which temporarily prevents movement of the workpiece contact element relative to the nosepiece, and thus maintains the combustion chamber in a closed position until the piston returns to the start position.